Cellular-type radio telephone systems have become established in metropolitan service areas, and numerous types of businesses have found that such systems can be used to improve customer service. Each service area is usually served by two cellular systems, by convention called A or B. Subscriber mobiles or portable units can discriminate between the two systems. The major metropolitan service areas are usually spaced far enough apart that neither A-type systems nor B-type systems overlap.
Attempts are now being made to install cellular-type systems with broader coverage areas in lower density rural areas. In these installations, A-type or B-type systems may overlap in fringe or border areas of coverage.
A cellular subscriber transceiver, such as a mobile unit or a personal portable unit, is designed to scan for the strongest control channel whenever a call is to be placed, at certain periodic time intervals when the transceiver is initially turned on, or if the signal being received goes below a certain level. If the transceiver is in an area served by more than one cellular system, there is a chance that the strongest control channel which the transceiver detects will be associated with the less desired one of the two systems.
As a result of the scanning process, the subscriber transceiver may lock onto the less desirable system, and place or answer its call there, even if the signal quality from the other system is adequate for a call to be made. This may cause the transceiver user to incur "roamer charges" that will increase the cost of the call. In addition, the revenue for the call is lost to the other carrier.
In the past, the solution to this problem was simple; the transceiver could be programmed to prefer system A or system B. When programmed to prefer system A, for example, the transceiver will lock onto a B system control channel only if no system A control channel is detected. This solution worked well, so long as no overlapping coverage was possible between two A-type systems or two B-type systems.
Currently, adjacent rural cellular systems can create this overlapping coverage. For this situation, prior solutions include:
1. Programming the subscriber transceiver for home only operation. This solution renders the transceiver unsuitable for roaming, though it does solve the overlapping coverage problem;
2. Installing a Type IV cellular repeater. In this solution, the repeater would provide a boosted control channel for the subscriber transceiver to lock onto, solving the overlapping coverage problem. However, such repeaters incorporate hardware to boost the reverse channel back to the donor cell, and require sufficient antenna isolation to operate efficiently; or
3. Installing an extra cell site with improved coverage and additional costs.
The prior solutions thus either limit subscriber service unacceptably, or provide other capabilities that raise their cost. There thus continues to be a need for cost-effective solutions to this problem which do not limit subscriber service.